By definition, radio communication systems contain antennas that serve as transducers between the radio frequency (RF) energy within the system and the radiated energy between systems. The antennas used must be compatible with the requirements of the particular RF system. While this results in a variety of designs for various communication systems, most antennas can be characterized by a common set of antenna performance parameters.
In order to select or design an antenna suitable for a given communication system, it is necessary to know, or be able to measure, the performance of the various alternatives. Typically, antenna performance parameters include input impedance, polarization, directivity, radiation efficiency, gain, and radiation pattern.
The transfer of RF power from an antenna to a communication system and vice versa is related to the input impedance of the antenna and the impedance of the communication system. The impedance match of the antenna and the system is thus of primary importance. The polarization of an antenna is defined as the polarization of the electromagnetic wave radiated by the antenna along a vector originating at the antenna and pointed along the primary direction of propagation. The directivity of an antenna is defined as 4 π times the ratio of the maximum radiation intensity (power radiated per unit solid angle) to the total power radiated by the antenna. The gain of an antenna is a measure of its ability to concentrate its radiated power in a particular direction. The radiation efficiency of an antenna is the ratio of the power radiated by the antenna to the net power accepted at its input.
Antenna radiation patterns are graphical representations of the directional distribution of energy radiated from the antenna. Radiation patterns can be plotted in terms of field strength, power density, or decibels. They can be absolute or relative to some reference level, with the peak of the beam often chosen as the reference. Radiation patterns can be displayed in rectangular or polar forms as functions of the spherical coordinates, θ and Φ;. It is fundamentally important to state that reciprocity applies, i.e. although the performance parameters of an antenna have been explicitly defined above in terms of the antenna as a source of radiation, the parameters (polarization, directivity, gain and radiation pattern) are the same whether the antenna is used for transmission or reception. Reciprocity is implicit in the descriptions and claims below; it is irrelevant whether a particular antenna is used for transmission or reception.
Antenna radiation-pattern measurements are traditionally performed using outdoor antenna ranges and indoor anechoic chambers. The choice of facility is influenced by a number of factors that include operating frequency, the size of the antenna or object to be measured, throughput or measurement speed, and the required measurement accuracy. An antenna range is a costly facility that requires specialist design and construction skills, space, regular maintenance and calibration, skilled personnel and resource management.
For both outdoor and indoor measurements of either the far-field or near-field components of an antenna's radiation pattern, it is important that neither the antenna under test (AUT) nor the range antenna (RA) itself are affected by unwanted signals or stray energy. Examples of stray energy include: interference from extraneous electrical, radio and microwave sources and reflections of the measured or wanted signal itself, hereinafter referred to as multipath.
When the AUT is used in the reception mode, accurate measurement of its far-field radiation pattern ideally requires zero variation in the amplitude and phase of the incident field at a given frequency across its aperture. For many measurements, however, a wave front with less than 22.5° of phase variation is considered sufficient. This is achieved by making the distance between the two antennas (the range antenna and the antenna under test) sufficiently large. Nevertheless, in practice it is desirable to make antenna measurements using a minimal separation distance consistent with obtaining the required accuracy. Small phase deviations result in minor distortions of the measured sidelobe structure while larger deviations cause major errors in the measured gain and lobe structure. This condition can also mask asymmetrical sidelobe structures that are actually present.
Large separation distances, as mentioned above, increase the probability of unwanted multipath reflections from the ground and other scatterers, reaching the antenna under test. Traditionally, in outdoor ranges, multipath reflections are reduced by employing one or more of the following methods: elevating the range antenna and the antenna under test; using directive range antennas; removing or reducing clutter and scattering surfaces; and adding screens or baffles to intercept reflected waves. An alternative procedure requires the use of a flat range and includes the effect of specular ground reflections.
In cases where the length of the antenna range is reasonably short, the entire range can be housed indoors in an anechoic chamber. The basic design criteria for both indoor far-field and near-field anechoic chambers are the same as those for an outdoor range. However, to eliminate, or at least minimise reflections, the surfaces of the chamber are covered with radio frequency (RF) or microwave absorbing material. The absorber is designed to reduce reflected signals over a specified range of frequencies. Amongst the many advantages offered by indoor testing are improved security, avoidance of unwanted surveillance and the elimination of meteorological and other environmental factors. These advantages have accounted for the recent trend towards more sophisticated indoor facilities that employ compact ranges or near-field probing systems.